Motor Theory and Nameplate Data

How AC Induction Motors Work

The squirrel-cage induction motor is the workhorse of commercial and industrial electrical systems. Understanding its operating principles lets you read a nameplate intelligently and size the branch circuit correctly under NEC Article 430.

When three-phase AC voltage is applied to the stator windings, the current in each winding peaks at a different moment in time. This phase difference creates a rotating magnetic field inside the stator. That rotating field induces voltage in the short-circuited rotor bars (the "squirrel cage"), which in turn produces rotor current and a rotor magnetic field. The interaction between the two fields generates torque, causing the rotor to spin.

Slip

The rotor never quite catches the rotating field. If it did, there would be no relative motion, no induced voltage, and no torque. The small speed difference between the synchronous speed of the rotating field and the actual rotor speed is called slip, expressed as a percentage. A typical loaded squirrel-cage motor runs at 2–5% slip. Slip increases as mechanical load increases. Synchronous speed depends on the supply frequency and the number of motor poles: a 2-pole motor on 60 Hz turns at 3,600 RPM synchronously; a 4-pole motor turns at 1,800 RPM synchronously.

Torque Curves and Power Factor

A torque-speed curve shows how much torque a motor produces at each speed from standstill to near-synchronous speed. Key points are locked-rotor torque (the torque at zero speed when the motor first starts), breakdown torque (the maximum torque before the motor stalls), and full-load torque (the rated operating point). Selecting a motor for a hard-starting load—a compressor, for example—requires adequate locked-rotor torque.

Power factor in a motor circuit is the ratio of real power (watts) to apparent power (volt-amperes). Induction motors draw lagging reactive current to maintain their magnetic fields. At full load, power factor may be 0.85–0.90. At light load it drops significantly, which is why oversized motors waste energy and may cause power-quality problems.

Reading the Nameplate

Every motor ships with a nameplate that is the primary source of truth for sizing conductors, overcurrent protection, and disconnects. Key fields:

• Voltage: The voltage(s) the motor is designed for. A dual-voltage motor may show "230/460 V." Always verify supply voltage matches nameplate before connecting.
• Full-Load Amperes (FLA): The current drawn at rated load and rated voltage. NEC Article 430 requires you to use NEC Table values (Table 430.247–430.250) for sizing conductors and overcurrent devices—not the nameplate FLA—unless the nameplate value is higher, in which case use the higher value.
• Service Factor (SF): A multiplier showing how much overload the motor can sustain continuously without damage. An SF of 1.15 means the motor can operate at 115% of rated horsepower under favorable conditions. Service factor does not change the branch-circuit sizing calculation.
• Insulation Class: Designates the maximum allowable winding temperature. Common classes for general-purpose motors are Class B (130 °C total) and Class F (155 °C total). Exceeding the rated temperature shortens winding life dramatically.
• NEMA Frame: A standardized designation (such as 56, 143T, 182T) that defines the motor's physical mounting dimensions—shaft height, bolt pattern, and shaft diameter. Frame standardization lets you replace a failed motor with any manufacturer's equivalent frame without re-drilling.
• Horsepower (HP) and RPM: Rated mechanical output and synchronous-based operating speed.
• Phase and Hertz: Confirms single-phase or three-phase design and the supply frequency.

Single-Phase Motor Types

Single-phase motors require a starting mechanism because a single-phase field does not inherently rotate.

• Split-phase: Uses a high-resistance start winding displaced electrically from the run winding. Simple and inexpensive; suited for easy-starting loads like fans.
• Capacitor-start: Adds a capacitor in series with the start winding to create a larger phase shift. Produces higher starting torque; common for compressors and pumps.
• Capacitor-run (permanent-split capacitor): Keeps the capacitor in the circuit continuously, improving running efficiency and power factor. Often used in HVAC equipment.

NEC Article 430 Sizing Rules

Article 430 governs motor branch circuits. The key relationships to memorize:

• Branch-circuit conductors: Sized at not less than 125% of the motor's full-load current from the applicable NEC table.
• Overload protection: Set at not more than 125% of FLA for motors with a SF of 1.15 or greater (or Class B or better insulation); otherwise not more than 115% of FLA.
• Short-circuit and ground-fault protection (overcurrent device): Sized per NEC Table 430.52, which lists maximum percentages of FLA for different protective device types. Inverse-time circuit breakers are typically allowed up to 250% of table FLA for general-purpose motors.
• Disconnect: Must be rated at least 115% of the motor's full-load current from the NEC table.

Safety Notes

Before working on any motor circuit, apply lockout/tagout (LOTO) per OSHA 29 CFR 1910.147. Verify the absence of voltage with a calibrated meter. Capacitor-start motors store charge in their capacitors; discharge capacitors safely before touching internal components. Wear appropriate PPE including safety glasses and insulated gloves rated for the circuit voltage. Never exceed the motor's nameplate voltage or allow it to run single-phase on a three-phase connection—both conditions cause overheating and potential fire.